In the manufacture of aircraft, ships, buildings, and other structures it is often desirable to use large sheets of aluminum or other materials for, e.g., exterior surfaces, or “skins.” Using larger sheets rather than riveting together multiple smaller sheets both reduces the number of fasteners and improves the performance of the structure. However, suppliers of aluminum sheets are limited in the sizes they can produce by milling technology. Thus, there is a need to join aluminum sheets together with base material-like properties.
Conventional welding is not suitable for use on high performance aluminum alloys (i.e., 2XXX and 7XXX) as it results in a joint having a heat-affected zone with changed heat treatment and corrosion issues. Friction stir welding can join aluminum, but it results in a stir zone with varying metallurgical properties, and it also creates a heat-affected zone that reduces performance.
Ultrasonic joining is a joining technique which causes dynamic recrystallization of new grains to form an interface. Ultrasonic joining can join aluminum but is limited in that the energy required to cause dynamic recrystallization must travel through the material, and as the material becomes thicker, the energy requirement increases exponentially. At some point, the energy required to cause the dynamic recrystallization damages the exterior, which imposes a limit on the thickness, approximately 0.04 inch, that can be joined. Some work has been done to successfully join sheets up to 0.07 inch thick by using ultrasonic joining on both sides of a scarf joint. However, ultrasonic joining has not been used to join sheets between 0.08 inch and 0.24 inch thick, which would be useful in the aircraft manufacturing industry.
This background discussion is intended to provide information related to the present invention which is not necessarily prior art.